The use of several mechanisms for controlling hydraulic actuators which are in turn used to position control surfaces on aerospace vehicles is well known in the art. In some instances the loads which are to be controlled are extremely heavy or have large forces applied thereto. Under certain conditions such loads have a resonant frequency which is within or very near the band pass of the hydraulic actuating systems. Means must therefore be taken to damp oscillations that may occur as a result of application of hydraulic power from a control valve within the system to the load or as a result of external forces applied to the load. This becomes particularly acute when the frequency of application of the power is near or approaches the resonant frequency of the load. In other instances the dynamic stiffness of the actuator must be adjusted to minimize the risk of surface flutter; usually requiring an actuator stiffness increase.
In the prior art, systems exist to achieve dynamic load damping of flight control actuators. However, such mechanisms typically require two moving spools both of which are spring centered and to make the apparatus function properly there must also be included properly adjusted metering orifices. Although apparatus of the type disclosed in the prior art have proven successful for the purposes intended it has been found that due to the complexity of the systems they are difficult to construct, adjust and maintain. Typical of such prior art systems are those shown and described in U.S. Pat. Nos. 3,138,072; 3,042,005; and 3,064,627.